Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  y : V.t
  Vy : ~ VSet.In y (V G)
  e : EdgeSet.elt
  Hle : leq_vertices G n e..s y
  nneg : n <= 0
  Vx : ~ VSet.In e..s (V G)
  Hxs : lsp G e..s (s G) = None
  n0 : e..s <> y
  H : EdgeSet.In e (E G)
  n1 : e..s <> y
  n2 : e..t <> y
  ============================
   (EdgeSet.In e (E G))


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"